Exchange of blood between pulmonary and systemic circulations via bronchopulmonary anastomoses.

• The existence of brouchopulmonary anastomoses has led to the general belief that the blood from the aorta and bronchial arteries can reach the pulmonary vessels and left atrium. There is reasonable evidence to suggest that the reverse can occur. Three observations pertain to the flow of blood from the pulmonary artery to the bronchial veins and right atrium: (a) In 1931 Berry and Daly perfused the entire pulmonary arterial and bronchial vessels of the dog and noted that in two dogs, about 1 per cent of the pulmonary arterial blood finds its way to the right atrium, (b) More recently, an isolated right lung preparation was perfused simultaneously through the pulmonary and bronchial vascular systems. In each of three dogs, the maximal contribution of pulmonary arterial blood to right atrial flow were 2, 8, 7.3, and 7.5 per cent of the total pulmonary blood flow, (c) The heart-lung preparation was modified so that the bronchial arterial flow was maintained and bronchial venous flow was collected. The total bronchial venous flow was reduced by 30 to 50 per cent when bronchial arteries were occluded. This suggests that under the conditions of these experiments, about 50 to 70 per cent of bronchial venous flow arose from blood in the pulmonary circulation.